Problem Statement
How do you implement authentication middleware in Next.js and what are best practices?
Explanation
Create a middleware.js file that checks authentication by reading cookies or tokens from the request, verifying them against your auth provider, and either allowing the request through, redirecting to login, or returning a 401 response. Use the matcher config to specify protected routes like matcher: ['/dashboard/:path*', '/api/:path*'] to run middleware only where needed, improving performance by not running on every request. Best practices include using the Edge Runtime for fast authentication checks, storing JWTs in httpOnly cookies for security, implementing proper error handling and logging, using NextResponse.redirect for login redirects, and caching auth checks when possible. For more complex auth, integrate with providers like Clerk, Auth0, or NextAuth.js that provide built-in middleware, handle token refresh automatically, and manage session state efficiently across your application.